Applied organic geochemistry provides the information needed to make maps of the richness, type, and thermal maturity of a source rock. These maps are a necessary step toward determining the stratigraphic and geographic extent of a pod of active source rock in a petroleum system, and they are based on geochemical analyses of rock samples from outcrops and wells that are displayed on logs. These geochemical well logs are based on Rock-Eval pyrolysis, total organic carbon, vitrinite reflectance, and other rapid, inexpensive “screening” methods. The logs define (1) potential, effective, and spent petroleum source rock; (2) the thermal maturation gradient, including immature, mature, and postmature zones, and (3) in situ and migrated petroleum shows. Useful geochemical logs require proper sample selection, preparation, analysis, and interpretation. Detailed studies, including oil-source rock correlations by biomarker and supporting techniques, are undertaken on selected samples only after the screening methods are completed.